1. Field of the Disclosure
The present disclosure relates to band-gap reference voltage circuits.
2. Description of the Related Art
As an example of a circuit that generates a reference voltage with low temperature dependence, a band-gap reference voltage circuit is known (for example, see Japanese Unexamined Patent Application Publication No. 2013-191095).
FIG. 4 is a diagram illustrating the typical configuration of a band-gap reference voltage circuit. A band-gap reference voltage circuit 400 includes an operational amplifier 110, resistors 120 to 122 and diodes 130 and 131.
One end of the resistor 120 is electrically connected to an output terminal of the operational amplifier 110 and the other end of the resistor 120 is electrically connected to a non-inverting input terminal of the operational amplifier 110. One end of the resistor 121 is electrically connected to the output terminal of the operational amplifier 110 and the other end of the resistor 121 is electrically connected to an inverting input terminal of the operational amplifier 110. An anode of the diode 130 is electrically connected to the non-inverting input terminal of the operational amplifier 110 and a cathode of the diode 130 is grounded. One end of the resistor 122 is electrically connected to the inverting input terminal of the operational amplifier 110 and the other end of the resistor 122 is electrically connected to an anode of the diode 131. A cathode of the diode 131 is grounded. The size of the diode 131 is around m times as large as the size of the diode 130.
In the band-gap reference voltage circuit 400, a band-gap reference voltage VBG is output from the output terminal of the operational amplifier 110.
The band-gap reference voltage VBG output from the band-gap reference voltage circuit 400 can be calculated in the following way.
The relation of the following expression (1) holds true between a voltage VA of the non-inverting input terminal and a voltage VB of the inverting input terminal due to the imaginary short circuiting of the non-inverting input terminal and the inverting input terminal of the operational amplifier 110.VA=VB   (1)
A forward voltage VF of a diode is expressed by the following expression (2).VF=VT×ln(I/IS+1)   (2)
Here, VT is a thermal voltage KT/q (where k is the Boltzmann constant, T is the absolute temperature and q is an elementary electrical charge), I is a forward current and IS is a reverse saturation current.
The reverse saturation current IS is very small compared with the forward saturation current I and expression (2) is approximated by the following expression (3).VF=VT×ln(I/IS)   (3)
When the resistances of the resistors 120 to 122 are respectively represented by R1 to R3, a parasitic resistance due to a wiring line between a point A (the connection point between resistor 120 and a diode 130) and the anode of the diode 130 is represented by RP, and forward currents of the diodes 130 and 131 are respectively represented by IA and IB, the following expression (4) is obtained from expression (1) and expression (3).RP×IA+VT×ln(IA/IS)=R3×IB+VT×ln(IB/mIS)   (4)
Here, if R1=R2, IA=IB and therefore the following expression (5) is obtained by replacing IA and IB in expression (4) with I.I=1/(R3+RP)×VT×ln(m)   (5)
Furthermore, the band-gap reference voltage VBG is expressed by the following expression (6).VBG=R2×I+R3×I+VT×ln(I/mIS)   (6)
As a result of substituting I in expression (6) with expression (5), the band gap reference voltage VBG is expressed by the following expression (7).VBG=(R2+R3)/(R3+RP)×VT×ln(m)+VT×ln(1/(mIS×(R3−RP))×VT×ln(m))   (7)
As illustrated in expression (7), the band-gap reference voltage VBG is affected by the parasitic resistance RP. FIG. 5 is a diagram illustrating an example of the relationship between the parasitic resistance RP and the band-gap reference voltage VBG. In the example illustrated in FIG. 5, a design value of the band-gap reference voltage VBG is around 1.23 V. If the parasitic resistance RP is about 40Ω, the band-gap reference voltage VBG is around 1.25 V. That is, the band-gap reference voltage VBG is shifted by around 20 mV from the design value.
The present disclosure was made in light of the above-described circumstances and an object thereof is to reduce an error in a band-gap reference voltage.